News Release Number: STScI-1993-13

The full news release story:

NASA's Hubble 'Space Telescope (HST) has discovered a new population of stars
isolated deep in the core of M15, one of the densest globular star clusters.

The stars are among the hottest stars observed in the core of a globular cluster. The
most likely explanation for their existence is that they are the "naked cores" of stars
that have been stripped of their outer envelope of gas, according to astronomers.

"This is amazing; these objects represent a totally new population of very blue stars,"
says Guido De Marchi, of the Space Telescope Science Institute (STScI), Baltimore,
Maryland, and the University of Florence, Italy. "When we started wondering what
they could be, we realized that they may be among the first observed stars to have
been stripped."

De Marchi and Dr. Francesco Paresce (STScI and European Space Agency) explain
that this could only have happened if stars are so crowded together in the core they
can be stripped of much of their gaseous envelopes by the gravitational pull of
bypassing stars.

This stellar cannibalism could only take place where stars are so crowded together
that chances for close encounters are exceptionally high, they said. De Marchi and
Paresce interpret the existence of this new class of stars as possible evidence that the
center of the globular cluster has contracted to an extremely dense condition called
"core collapse."

This research, by De Marchi and Paresce, is being announced at a press conference
today at the meeting of the American Astronomical Society in Berkeley, California.

The astronomers were surprised to discover about 15 hot blue stars segregated at the very core.
Their surface temperatures are above 60,000 degrees Fahrenheit (our Sun's surface is 11,000
degrees Fahrenheit). This discovery was possible only with the Hubble Space Telescope because
it can resolve stars at the dense core of M15 that are only a blur from the ground. The
observations also required Hubble's sensitivity to ultraviolet light to distinguish the hot stars
from the surrounding cooler stars.

Such very hot blue stars can be made in several ways besides stellar stripping: magnetically
stirred-up super massive stars, white dwarfs, or planetary nebulae. However, the researchers are
quick to point out that none of these scenarios explain why the stars are so concentrated and so
numerous only at M15's core.

"This rules out a number of other hypotheses," says De Marchi. He explains that all the blue stars
lie within a one light-year radius at the very core of the cluster. What's more, 90 percent of them
are concentrated at the very center of this volume, within a four-tenth light-year radius.

Close Encounters of the Stellar Kind

According to this scenario, the new population of blue stars was once the cores of red giant stars.
Such stars expand to enormous sizes late in their lives, due to changes in the nuclear "burning" at
their cores. (If our sun were the size of a red giant it would engulf the inner solar system, out to
the diameter of Mars' orbit.)

Red giant stars are so distended that they have a weak gravitational hold on their outer envelope
of cool gas. If a normal, main sequence star passes within a few stellar radii it can rob gas from
the red giant. This stripping process can, in theory, expose a star's core  the nuclear fusion
"engine' that powers stars.

However, conditions where stars are so crammed together are unusual. For example, in our own
stellar neighborhood, the stars are typically a million times farther apart than the distance
between the sun and Earth. Conversely, due to the relentless pull of gravity, the stars at the core
of M15 have converged so that they are at about 500 times the distance between the Earth and
the sun. If our planet were there, we would see a hundred thousand stars closer than Proxima
Centauri, the closest star to the Earth; the night sky would look simply fantastic!

The astronomers used Hubble Space Telescope's Faint Object Camera (FOC) to probe the core
of M15 (15th object in the Messier Catalog) which is located 30,000 light-years away in the
constellation Pegasus. M15 is visible to the naked eye as a hazy spot 1/3rd the diameter of the
full moon.

Core Collapse

Globular clusters are compact "beehive swarms" of several hundred thousand stars loosely held
together under the mutual pull of gravity. The stars are deflected by gravity if they pass near each
other. During such close encounters a smaller, less massive star steals momentum from the larger
star.

Because of these near-collisions, the massive stars lose momentum and "fall" toward the center
of the cluster, like marbles rolling to the bottom of a funnel. Given enough time, massive stars
should accumulate at the cluster's center. Theoretically, this could become a runaway collapse
where stars quickly crowd together.

Previous Hubble observations suggest that the cluster probably contains powerful energy
"storage batteries" in the form of double star systems, which prevent the core from imploding all
the way down to a black hole. The rapid orbits of two stars about each other in tight binary
systems create a powerful reservoir of kinetic energy. A few double stars can stir up the motion
of in-falling stars. This would cause the core to rebound, like squeezing and relaxing a rubber
ball.

Astronomers have long sought evidence for core collapse at the heart of very dense clusters like
M15. To estimate the true stellar density from ground-based visible light photographs, however,
has been difficult. The Hubble observation doesn't tell whether the core is still collapsing or
rebounding.

Previous research by a team led by Paresce found that another class of unusual blue star, dubbed
blue stragglers, also dwells at the cores of some clusters. However, even the "stragglers" are not
as hot nor as blue as the new population of blue stars in M15. Most of the blue stragglers are
probably double stars that gravitationally capture each other. The capture stirs-up the stragglers'
nuclear fuel. The star "resets its clock" to relive a bright and hot youth.

The researchers plan to use Hubble to peer into the cores of other globular star clusters to see if
this new class of star dwells elsewhere as well.